DIGITAL EQUIPMENT CORPORATION • MAYNARD. MASSACHUSETTS
DIGITAL EQUIPMENT CORPORATION • MAYNARD. MASSACHUSETTS
1st Printing November 1966
2nd Printing February 1967
3rd Printing June 1967
4th Printing October 1968
5th Printing November 1968
6th Printing May 1969
Copyright (C) 1969 by Digital Equipment Corporation
Instruction times, operating speeds and the like are in-
cluded in this manual for reference only; they are not to
be taken as specifications.
The following are registered trademarks of Digital
Equipment Corporation, Maynard, Massachusetts:
INTRODUCTION AND DESCRIPTION
1.1 General Description 1-1
1.2 Scope of Manual 1-2
1.3 Pertinent Documents 1-2
1.3.1 Manuals 1-2
1.3.2 Engineering Drawings 1-2
1.3.3 New Module News Bulletins 1-3
1.4 Functional Description 1-3
1.5 Physical Description 1-4
1.5.1 Electrical Details 1-6
1.6 TU55 Performance Characteristics 1-9
THEORY OF OPERATION
2.1 Block Diagram Analysis 2-1
2.2 Detailed Descriptions 2-4
2.2.1 Interface 2-4
2.2.2 Unit Select Lines 2-5
2.2.3 Command Lines 2-5
2.2.4 WRITE ENABLE Signal 2-5
2.2.5 Interface Connections 2-6
2.2.6 Read/Write Head 2-6
2.2.7 Tape Motion Control 2-6
2.2.8 Remote or Programmed Control 2-6
2.2.9 Local (Manual) Control 2-7
2.2.10 Motor Control 2-7
2.2.11 Transport Selected Signal 2-8
2.3 Module Descriptions 2-9
2.3.1 Type R303 Integrating One Shot (Delay) 2-9
2.3.2 Type W513 Level Amplifier 2-10
2.3.3 Type G850 SCR Motor Driver 2-11
Controls and Indicators
Tape Tension and Transport Stop Adjustment
Head Output Check
Write Enable Circuit Check
Logic Signal Symbols
FLIP CHIP Pulses
Type TU55 DEC tape Transport
Hub and Reel Assembly
Arrangement of DECtape Head
Type TU55 DECtape Transport, Rear View
1-5 TU55 Interface Connections 1-8
2-1 DECtape Transport TU55 Block Diagram 2-1
2-2 Head Connections 2-4
2-3 Schematic of R303 Integrating One-Shot 2-10
2-4 Schematic of G850 SCR Motor Drive 2-1 1
4-1 Module Adjustment Trimports 4-3
5-1 DEC Symbols 5-2
5-2 FLIP CHIP R-Series Pulse 5-4
5-3 FLIP CHIP B-Series Pulse 5-4
Solid State DECtape Transport, TU55-0-2 5-5
Module Utilization List, TU55-0-5 5-7
Bus Bar for TU55, TU55-0-4 5-7
Relay, G851 5-8
Diode Cluster, R002 5-8
Inverter, R107 5-9
Diode Gate, Rill 5-9
Dual Flip-Flop, R202 5-10
Integrating One-Shot, R303 5-10
Solenoid Driver, W040 5-11
Level Amplifier, W513 5-11
1-1 Summary of Equipment Characteristics for the TU55 DECtape Transport 1-9
2-1 Motor and Brake Operation Truth Table 2-9
3-1 Functions of Controls and Indicators 3-2
4-1 Recommended Maintenance Equipment 4-1
4-2 Recommended Spare Parts 4-7
5-1 Semiconductor Substitution 5-4
INTRODUCTION AND DESCRIPTION
The Type TU55 DECtape Transport (see figure 1-1) is a solid-state, bidirectional, magnetic-
tape handlingdevice designed and manufactured by the Digital Equipment Corporation (DEC) for use in
DEC digital computer systems. When used with a suitable DECtape control system, the 11)55 provides a
fixed-address magnetic-tape facility for high-speed loading, readout, and program updating. The DEC-
tape control system directs the transport to read forward or in reverse, to write forward or in reverse, to
stop, and to go. The TU55 contains tape handling elements, drive mechanisms, and solid-state switching
circuits which switch the tape head onto a master bus system and interpret command instructions from the
control unit. The highly reliable solid-state switching circuits in the TU55 are completely compatible
with the Type 555 DECtape Transport (which uses relay switching) and may be used to modify and
expand systems employing this type of transport.
Figure 1-1 Type TU55 DECtape Transport
1.2 SCOPE OF MANUAL
This instruction manual is intended to aid personnel in the maintenance of the 11)55 DECtape
Transport. The equipment is discussed primarily from a maintenance point of view, but some information
is given on the operation of its associated controls and indicators. The documents listed below provide
operational information required for programming.
Since the transport is one element of a computer/control/transport system, the reader should
know the basic internal operations of both the computer and control . Of special importance are the
sections concerning program interrupts, programmed in/out transfers, and the databreak facility in the
computer; and the select and motion control logic in the external DECtape control. Detailed descrip-
tions of the tape format, instruction repertoire, and programming practice for using the TU55 with a
particular computer is described in the user handbook for the computer. Control information for the
TU55 can be obtained from the maintenance manual for the DECtape control system used with the
1 .3 PERTINENT DOCUMENTS
The following documents provide source information relative to the use of the DECtape Trans-
port Type TU55.
Digital FLIP CHIP Modules Catalog, C-l 05
PDP-7 User Handbook, F-75
PDP-8 User Handbook, F-85
DECtape Control 550 Instruction Manual, H-550
DECtape Control 551 Instruction Manual, H-551
DECtape Control 552 Instruction Manual, H-552
DECtape Control TC01 Instruction Manual, DEC-08-I3AB-D
DECtape PDP-8 Programming Manual, Digital-8-27-U
1 .3.2 Engineering Drawings
A set of reduced engineering drawings for the TU55 is contained in chapter 5. These drawings
are in addition to the complete set of full-size drawings forwarded with each TU55. As explained in
chapter 5 all maintenance personnel should use only the full size engineering drawings for work on the
equipment because these drawings show variations peculiar to an individual installation.
1 .3.3 New Module News Bulletins
G850 SCR Motor Driver
W513 Level Amplifier
In addition to the above documents, complete sets of Library Programs are available for each
computer using DECtape systems.
1 .4 FUNCTIONAL DESCRIPTION
The TU55 Transport provides a read/write head for recording and playback of information on
five channels of the 0.75-inch tape. Each channel consists of two nonadjacent coils which are wired
in series. Thus, information on one track combines with redundant information on another to create a
single signal. Connections from the read/write head are made directly to the external control unit which
contains the read and write amplifiers as well as the command logic for the selection and remote control
of tape motion.
The left half of the front panel (see Figure 1-1) contains the tape deck with reels and reel
motors, tape guides, and the read/write head; the right half is the local control panel with rocker
switches for selection and manual operation. The 600-rpm induction motors that drive the reel hubs
directly control tape motion. No capstans, pinch rollers, or drag pads contact the tape.
The logic circuits of the TU55 command tape movement in either direction over the read/write
heads. Solid-state switching circuits completely control the tape drive motors. These circuits govern
the torque applied to each reel motor to transport the tape across the head in accordance with specific
commands (i.e., go, forward, reverse, stop).
In normal tape movement, full torque is applied to the take-up reel, thus establishing the
direction of motion. Reduced torque, applied to the trailing reel, maintains proper tape tension. Tape
motion is bidirectional so that either reel can serve as the take-up reel. The DECtape system uses the
so-called Manchester phase recording technique rather than an amplitude sensing technique; thus, tape
speed need not be a precisely controlled parameter. Actually, the speed varies ±20%, depending upon
the diameter of the tape pack on the take-up reel . An electromagnetic brake mounted on each motor
shaft achieves a positive stop by braking the trailing reel at the end of a motion command. The take-up
reel motor continues to apply partial torque to take up tape slack. Whenever the tape is motionless,
one of the two reel brakes is applied opposite to the direction of the last tape movement.
Tape movement is controlled either by commands originating in the computer and applied to
the TU55 via a suitable DECtape control system, or by commands generated through manual operation
of rocker type switches located on the front panel of the transport. Typical DECtape control systems
which allow transfer of information between the computer and the TU55 are as follows:
Computer Typical DECtape Control System
PDP-1, -4 and -7 Type 550
PDP-6 Type 551
PDP-5 and -8 Type 552
Manual control is used to mount new reels of tape on the TU55 or as a quick maintenance check for
proper operation of the control logic in moving the tape.
External DECtape control systems may control up to eight individually addressed TU55s to
read or write tape. The operator may select the address of each drive by adjusting the thumbwheel
selector at the center of the TU55 local control panel (see figure 1-1). The operator may also place
the drive off-line by a setting on the same thumbwheel or by switching the drive for local operation.
In local operation, the head is disconnected and the rocker switches on the local control panel regu-
late tape motion .
1 .5 PHYSICAL DESCRIPTION
All components of the TU55 are mounted on a preformed and assembled chassis. Roller slides
that allow easy access to the top, sides, and rear of the drive hold the chassis in a standard DEC bay.
Double doors in the front and rear are held closed by magnetic latches. Power supplies and controls
are mounted on the rear of the DEC bay. Generally, the transport is mounted with others in the same
bay that contains the DECtape control system.
The tape deck is machined from 3/8-inch cast aluminum plate and held to the chassis by cap
screws at the corners. Reel motors, tape guides and the head are secured only to the deck. This
arrangement preserves the integrity of the tape alignment by the rigidity of the deck plate. Heavy
extruded aluminum plates at the top and bottom front of the chassis serve as bumpers to protect the
deck and control panel. The head is mounted in direct contact with the deck and is secured at both
sides by brackets. Head azimuth is set during manufacture of the drive and cannot be adjusted in the
No attempt should be made to loosen the brackets and change the head
Reel motors are held by four cap screws (hidden by the reel hubs) from the front of the deck.
At the rear of the shaft (which extends from both ends) is the electromagnetic brake. At the front, the
shaft has a flat for securing the reel hub by a single Allen-head set screw. The hubs are flanged qt the
rear and have an annular spring resting in a slot cut into the reel circumference. Beneath the spring in
the slot is a rubber ring seal which causes the spring to protrude above the slot to secure the reel (see
figure 1-2). The reel is a 1 -piece mold of phenolic composition. The inside circumference at the back
of the reel fits the hub. In front, however, the inside circumference is slightly larger, allowing the
hub spring to expand outward and to secure the reel. The clearance between the rear of the reel hub
and the tape deck is nominally 15 mils, but may vary on individual machines to guarantee proper tape
alignment in the guides. The guides are machined from aluminum and have a curvature designed to
maintain air-film lubrication between the tape and guides as long as tape is in motion. A fan mounted
on the top chassis plate continuously blows cooling air over the reel motors.
Figure 1-2 Hub and Reel Assembly
The redundantly paired tracks that form the five channels on the head are illustrated sche-
matically in figure 1-3. Three track pairs are used for data; the remaining pairs are used as timing and
control information. The two timing tracks are on the outside at opposite sides of the tape. Just inside
these are the two tracks for the mark channel, the contents of which signify to the external control the
type of information in the data channels. The six tracks for these data channels are grouped at the
center, where they are least affected by skew. Since writing may take place in the data channels
while reading takes place simultaneously in the timing and mark channels, the tracks for the latter
channels are separated slightly from the data channel tracks to allow room for additional shielding.
Figure 1 -3 Arrangement of DECtape Head
The TU55 requires the DEC standard +10 and -15 vdc power levels at the terminal-tab con-
nectors on the rear panel (figure 1-4). These dc levels are normally provided from power supplies
associated with the DECtape control system. AC power (105-125v, 60 cps) is connected at the 3-
terminal receptacle on the rear panel.
Figure 1-4 Type TU55 DECtape Transport, Rear View
Signal connections to and from the DECtape control system are made through 18-pin and
36-pin FLIP CHIP cable terminators that plug directly into the module mounting panel on the rear
panel (Figure 1-4). Cable locations in this mounting panel are shown in Figure 1-5 with respect to the
other installed modules. The command cable terminator is a Type W023 18-Pin Connector for the logic
signals governing selection and tape motion. The head signal connector is a double-size Type W032
36-Bit Connector for shielded cable. Two sockets at the TU55 logic panel are reserved for the command
connector and two for the information connector. The pins for each pair are bussed together. This is
done so that TU55s may be grouped. Each TU55 receives its information and commands from one trans-
port in the group through one set of connectors and sends these to the next transport through another set
The writing current is 210 ma. Playback peak-to-peak channel voltage is between 10 and
12 mv when the tape is up to speed.
Figure 1-5 TU55 Interface Connections
Command signal levels from the DECtape control to the TU55 Transport are either ground or
-3v. (Equivalent signals to a 555 Transport are either -3 or -15v.) For existing systems with a con-
trol system for the 555 Transport, exchanging a single FLIP CHIP module (a W513 Level Amplifier when
the 11)55 Transport is connected to a control designed to drive relay Transport 555; otherwise a W990
Blank Module) allows the TU55 to accept command signals of either type.
The reel brakes and the head relay are energized by — 15v through special solenoid driver
circuits; the motors are controlled by silicon controlled rectifier (SCR) circuits that phase-switch the
1 10 vac line power.
The TU55 uses twelve types of FLIP CHIP logic modules (connectors excluded) as follows:
2 W040 Solenoid Driver
1 W513 Level Amplifier/W990 Level
1 R107 Inverter
4 Rill Diode Gate
1 R303 Integrating One -Shot
1 R002 Diode Network
2 R202 Dual Flip-Flop
1 R303 Integrating Delay
2 G850 SCR Motor Driver
1 G851 Relay Module
3 W023 Indicator Amplifier
1 W701 Input Network
All modules are mounted on a dip-soldered epoxy board with 18 gold-plated contacts at one
end (36 on double-height modules) for connection to the mounting panel receptacle, and a plastic handle
bearing the type number at the other end. Three contacts on each module are reserved for supply voltages:
A and B supply the +10 and -15v standard power levels; C is the ground.
1 .6 TU55 PERFORMANCE CHARACTERISTICS
A summary of the characteristics of the 11)55 equipment is given in table 1-1 .
TABLE 1-1 SUMMARY OF EQUIPMENT CHARACTERISTICS
FOR THE TU55 DECTAPE TRANSPORT
10-1/2 in. high, 19-1/2 in. wide, 9-3/4 in. deep
Standard 19-in. rack. Four ^1 0-32 screws mount chassis
track assembly which holds transport.
Chassis can be extended 16-3/4 in. beyond mounting
surface for maintenance
— 15 vdc, 1 .0 amp maximum
+ 10 vdc, 50 ma maximum
1 15 vac ±10%, 1 .0 amp idle, 2.0 amp maximum current
(60- and 50-cycle models)
Commands: two I8-terminal FLIP CHIP female
Information: two 36-terminal FLIP CHIP female
Internally mounted fan
50 to 110°F ambient
10 to 90% relative humidity
NOTE: The manufacturer of the magnetic tape for DECtape recommends 40
to 60% relative humidity and 60 to 80°F as acceptable for operating environ-
Reel Diameter Ratio
260 ft of 3/4 in., 1 mil thick Mylar sandwich tape
2-3/4 in. empty reel, 3-3/4 in. for 260 ft of tape
Approx,. 1:4 (maximum to minimum)
TABLE 1-1 SUMMARY OF EQUIPMENT CHARACTERISTICS
FOR THE TU55 DECTAPE TRANSPORT (continued)
Tape Characteristics (continued)
Direct drive hubs and specially designed guides which
float the tape over the head hydrodynamically . No
capstans or pinch rollers are used.
93 ±12 ips
350 ±55 bits per inch
2.7 x 10 bits per reel assembled into computer-length
words by external DEC tape control
Times given are for 90% full speed.
Turn Around Time
<1 50 msec
<1 50 msec
Input Signals to Transport from Control
/ground level assertion,
y normally complementary levels
f ground level assertion,
y normally complementary levels
negative level assertion used to stop
transport when computer halts
SELECT 1 through SELECT 8 (the selected line will
be at ground)
Ou tput Signal from Transport to Control
WRITE ENABLE (standard DEC ground level assertion)
*Valid only when the control is operating the solid-state Transport TU55. When relay type transport
(Type 555) signals are received a conversion is made to DEC standard levels by the W513 Level
Amplifier which converts the — 3v level to ground and the floating input to — 3v.
THEORY OF OPERATION
BLOCK DIAGRAM ANALYSIS
The TU55 logic is shown in the functional block diagram of figure 2-1 . All front panel con-
trols and indicators are shown within the blocks representing the associated logic. The functions of these
controls and indicators are summarized in table 3-1 . Diamonds indicate the direction of signal flow be-
tween blocks; an open diamond represents a signal effective (asserted) at ground; a closed diamond rep-
resents a signal asserted at — 3v. All interface signals are received from or transmitted to an external
DECtape control system. One of the two connectors at the left of figure 2-1 serves as a command and
information signal bus to other TU55s.
DELAY ( 1 )
DIRECTION ( 1 )
DATA t (4)
DATA 1 (4)
DATA 2 (4)
^DATA 3 (4)
DELAY ( 1 K
AND I — , I ♦£ 1
-* — ♦
NOTE: An arrow signifies a nonstandard DEC signal, such as motor voltage,
head playback, brake voltage, etc.
In the motion control block the arrow above a switch means that pressing this
switch results in tape movement in the indicated direction when the middle
switch is in the LOCAL position.
Figure 2-1 DECtape Transport TU55 Block Diagram
The SELECTOR LOGIC block in figure 2-1 includes the front panel thumbwheel selector
shown in figure 1-1 . This selector has nine positions (designated 1 through 8 and OFF LINE) for selecting
one of eight select lines which are the outputs of a binary-coded-decimal decoder in the external DEC-
tape control system. During addressing, only one of the eight input select lines is at the asserted
ground. In remote operation, the SELECT output is asserted negative whenever the thumbwheel setting
corresponds to the asserted address line from the DECtape control system.
Writing is possible on a selected drive only when that drive furnishes control with a WRITE
ENABLE signal at ground. A selected TU55 furnishes such a signal when the WRITE ENABLE switch in
figure 2-1 is on. When this switch is set to WRITE LOCK, the output line is negative and writing can-
not take place. If the control is commanded to perform a write function when WRITE LOCK is selected,
it displays a selection error flag to notify the program of the mistake.
The five motion commands from the external DECtape control system are shown entering at
the left center. The ALL HALT signal is asserted negative by the external control whenever the computer
program execution is halted either by the program or by the operator. When the program is halted, the
computer cannot issue commands to peripheral equipment. The ALL HALT signal stops those transports
that were in motion at the time of the ALL HALT command, thus preventing a complete run-off of the
The other four commands, GO, STOP, FORWARD, and REVERSE, are asserted at ground by
the external control in response to computer program commands. In current DECtape controls, GO and
STOP are complementary levels. In the following discussion, the GO, STOP, FORWARD, and REVERSE
lines referred to are at the output of the signal interface module (B07).
Tape motion begins at a selected TU55 when the GO line is asserted (STOP must be held
false throughout the desired interval of tape motion). Motion stops whenever the STOP line is asserted
while the GO line is simultaneously held false.
The same conditions as above apply to the FORWARD and REVERSE lines. For a selected
transport which is set to REMOTE, asserting the FORWARD line (while holding the REVERSE line false)
causes all subsequent remote GO commands to move tape forward. Similarly, asserting the REVERSE
line (while holding the FORWARD line false) selects the reverse direction. In LOCAL operation, the
reverse and forward tape-motion switches override the last REMOTE direction command so that the tape
always moves in the direction of the arrow above the switch (see figure 2-1).
Current DECtape controls generate the GO/STOP and the FORWARD/REVERSE signals as
complementary levels. Forward tape motion, for example, is commanded by assertion levels on GO and
FORWARD accompanied by false levels on STOP and REVERSE.
When forward motion is commanded either locally or remotely, the motion control logic asserts
the FWD output negative. This signal releases both brakes and applies full clockwise torque to the right
motor and partial counterclockwise torque to the left motor. As a result the tape moves from the left
reel across the head onto the right reel. The REV line (asserted negative in response to the REVERSE
motion command) applies full torque to the left motor, partial torque to the right motor, and simulta-
neously releases both brakes. Tape motion, therefore, is from right to left. At the end of either com-
mand, the STOP line is asserted negative and triggers the stop delay (about TOO msec). For the
duration of the delay, full torque is applied to the trailing motor and stop torque is applied to the
leading motor (see section 3.3). Thus, for the duration of the delay after the STOP command has
been issued, the motor torques are reversed causing tape movement also to be reversed. This time,
however, is very short and when the delay times out, the motors come to zero speed while changing
their rotation from one direction to the other. After the STOP DELAY times out, the tape becomes
motionless and partial torque is applied to both reels to take up the tape slack and the brake is applied
to the motor shaft which was trailing during the previous motion. The effect of the delay is nullified
if a new motion command is given before the end of the 1-sec interval. Then, the torque signals to the
left and right motor drives are immediately reestablished according to the new motion command. The
delay starts again at the leading edge of each STOP signal regardless of the frequency of its occurrence;
i.e., no recovery time is inherent in this type at delay.
An illustration of the tape head is shown in figure 2-2. Every channel consists of two tracks
associated with their coils. When writing, the current flows from ground (center tap) through one of
the coils (to either + or — terminal)to saturate the tape in one direction and through the other coil to
saturate the tape in the other direction. The coils for the two tracks are wired in series and are brought
out as four lines: ground, +, — , and the shield. Within the head, relay module, the +, — , and ground
lines (center taps) for a channel are switched by one of five 3-pole, normally open reed relays. Shields
are not switched, but are through-connected from the head to the interface connectors. A separate
line from the timing channel (also shown in figure 2-2) is brought out to allow measurements of head
skew (or the head perpendicularity). This line isusedonly for test purposes (and not by the external
control). The single head-ground line is used to ground the head case.
Figure 2-2 Head Connections
This section describes, in detail, the flow of information and control through each of the
functional elements of the TU55 control unit. The discussion is limited to describing the various
modules as logical operators. Detailed descriptions of individual circuits are given in section 2.3,
The basis for the discussion in the remainder of this section is the block schematic in DEC
drawing BS-D-TU55-0-2. This drawing is subdivided into zones bounded by horizontal section A
through D and vertical section 1 through 8. Circuit locations are referenced by a letter-numeral
combination. For other drawing symbology, refer to the introductory paragraphs of chapter 5.
All interface signals received or transmitted by the tape motion control circuits are trans-
ferred between the TU55 and the external DECtape control system. Interface signals for the tape-
motion control logic consist of: eight unit select input lines, five command input lines, and selected
write enable output control lines. Bus connections for these signals from the DECtape control system
fro the TU55 are made by plugging a FLIP CHIP cable connector into module receptacles (A5 or A6).
These signals together with the power lines and the information lines (to be described under Read/Write
Head Circuits) constitute the interface for the entire TU55.
2.2.2 Unit Select Lines
The eight unit select lines, numbered 1 through 8, are the outputs of a binary-coded-decirnal
decoder in the DECtape control and are supplied to the TU55 for application to the unit selector switch
S5 (A7, A3). This manually operated switch establishes the programmed address of the TU55, to allow
the TU55 to be addressed by the computer program. The unit select lines are connected directly to the
corresponding numbered position contacts of the selector switch. The switch wiper arm is connected to
input terminal R of the module B07 (C8), thus providing the SELECT command as an output at terminal
B07. Module B07 is either a Type W513 Level Amplifier or a Type W990 Blank Module with appropriate
jumpers, depending upon the type of input signals. As previously explained, when the input levels are
— 3v and ground (i.e., the control has been designed specifically to drive the solid-state drive), the
W990 Blank Module is used. When an older type control is used (such as 550, 551, or 552) the W513
Level Amplifier is used instead to convert relay driving levels to DEC standard levels. The same applies
also to the command line level.
2.2.3 Command Lines
The four ma|or command lines: GO, STOP, FWD, and REV set or clear the MOTION or
DIRECTION flip-flops (C7, C6) based upon programmed commands from the computer.
The ALL HALT command level is generated in the control logic of the computer and occurs
when the program is halted. This negative level for assertion clears the MOTION flip-flop (C4) to the
motion(O) state to stop motion of the transport when the computer program halts. Obviously when the
computer program is halted, the computer connot Issue commands to peripheral equipment, and so this
action is taken to prevent indefinite running of the transport and to prevent the tape from running off
the reel. However, toenable the transport to run under local (or manual) mode, the ALL HALT level is
ANDed with the remote level (C4).
2.2.4 WRITE ENABLE Signal
The TU55 generates the ground level WRITE ENABLE signal for routing to the DECtape con-
trol from terminal board A5, pin L. To assert this level the WRITE ENABLE/WRITE LOCK switch S3 (A6)
must be in the WRITE ENABLE (up) position. This position enables diode gate VUT of module Bl 1 (B6)
to generate the WRITE ENABLE signal, and causes the lighting of the WRITE ENABLE indicator on the
For the older type controls, the W513 Level Amplifier circuit serves as a protection diode
for the R107 (B1 1 ) Inverter.
2.2.5 Interface Connections
Information lines are brought to the read/write head from the DECtape control system where
they are bussed together for connection to other transports. Connections to this bus from an individual
TU55 are made by cable connections of module connectors (A4 and B4). These connections are wired to
the Type G851 Relay Module (A4, B4) for connection to the read/write head when the relay is operated
by the (SELECT) signal amplified by solenoid driver ER of the Type W040 Module (B4).
2.2.6 Read/Write Head
A type G851 Relay Module (A4, B4) establishes connection to the read/write head from the
information bus lines of the DECtape control system. This module consists of 5 x 3 single-pole, normally
open contacts which are closed when the relay is energized. The relay is energized when the TU55 is
selected, thus connecting the read/write head channel coils to the control.
2.2.7 Tape Motion Control
Tape movement and direction of movement are controlled by two Type R202 Flip— Flops; namely,
stop/go MOTION flip-flop (C6) and a reverse/forward DIRECTION flip-flop (C7). The flip-flops are
set or cleared by command signals originating in the DECtape control system or in the reverse tape-motion
and forward tape-motion switches on the front panel (see Figure 1-1).
2.2.8 Remote or Programmed Control
The four major commands that originate in the DECtape control system are GO, STOP, FOR-
WARD, and REVERSE. Type Rill Diode Gates (B4, B5, B6) NAND-and combine these commands with
the SELECT level within the TU55. The output of these gates is used to set and clear the MOTION and
DIRECTION flip-flops. Therefore, motion and direction are controlled only in the selected transport.
The TU55 receives these four major commands as direct connections from the DECtape control system.
The eight unit select lines, designated 1 through 8, (B8) are connected to appropriate positions of the
unit selector switch (A7, A8). The transport is selected when the selected line coincides with the
position of the unit selector switch. The negative SELECT level is conditioned by LOCAL and SELECT
ground levels (B7) if the transport is switched to the local mode. Then computer-control selection of
the transport is not affected.
The magnitude of the load on the address line is examined by the external control during
selection to determine whether more than one DECtape transport is being addressed.
2.2.9 Local (Manual) Control
Switches on the front panel of the TU55 provide for manual control of the tape motion. The
shift in control from remote to local is performed through the REMOTE/OFF/LOCAL switch S2A (A6).
With the switch on LOCAL, the selected level (C7) is forced to ground potential thus inhibiting the
external command NAND gates (Rills). Inhibiting these gates prevents commands from the DECtape
control system from affecting the state of either the MOTION or DIRECTION flip-flops and thus pre-
vents interference with manual control. With switch S2 on LOCAL, +10v is supplied to both the
REVERSE S4 (A5) and FORWARD SI (A7) switches. This condition permits the switch operation to es-
tablish the appropriate state of both the MOTION and DIRECTION flip-flops when pressed. Switches
SI and S4 are of the momentary contact type which return to a reset position when released. When the
REVERSE switch is pressed, the MOTION flip-flop is set to the motion(l) state and the DIRECTION
flip-flop is set to the direction(O) state by grounding the appropriate output connection terminals of the
flip-flops. The grounding of the flip-flop outputs is done via the outputs of the W701 Switch Filter,
generally used to interface switch action to the logic (refer to the circuit description). This module is
used both to filter out switch contact bouncing and to load the same contact with some current from a
sufficiently high potential to positively bridge across any insulating film that might be formed over the
switch contacts, thus assuring an electrical contact when the switch is pressed.
Under these conditions the tape moves in reverse. When the FORWARD switch is pressed,
the MOTION flip-flop is set to motion(l) and the DIRECTION flip-flop is set to direction(l), moving
the tape forward. Release of either the REVERSE or FORWARD switch clears the MOTION flip-flop,
thus stopping tape motion. Therefore, tape motion under manual control occurs only as long as the
REVERSE or FORWARD switch is held down physically. Inter-wiring between the REVERSE and FORWARD
switches provides a protection feature, so that if both switches are pressed simultaneously, the FORWARD
direction will prevail.
2.2.10 Motor Control
The two motors which move the tape past the read/write head are operated by ac power which
is controlled separately for each motor by an SCR switch on a Type G850 SCR Motor Control Module
(D4, C4). These SCR circuits, in turn, are controlled by combinations of the states of both the MOTION
and DIRECTION flip-flops. A brake on the shaft of each motor is also individually controlled by com-
binations of the state of these control flip-flops (C7, C6). The states of the control flip-flops are
decoded by Rl 1 1 NAND gates which control the SCR circuits, and by Type W040 Solenoid Driver
Modules (D4, C4) which control the brakes. Table 2-1 indicates the assertion levels (1) which operate
the motors and brakes; or indicates the nonassertion levels (0) which do not operate these devices for
all combinations of control flip-flop conditions.
Table 2-1 also presents the three states of motor and brake operation when the tape is
running in either the forward or reverse direction.
The functions of the Type G850 SCR Motor Driver Modules (D4, C4) are as follows: with a
negative assertion level at terminal H, the motor is driven at full torque; with a negative assertion level
at terminal F, the motor is driven with enough drag torque to take up slack in the tape when the motor
functions as a trailing motor; with a negative assertion level at terminal E, the motor has just enough
torque to take up slack in the STOP condition.
When the tape is running in the forward direction, full torque is applied to the right motor
and drag torque is applied to the left motor. When the STOP command is given, full torque is removed
from the right motor and applied to the left motor for the duration of the delay of the Type R303 One
Shot Module (07), and stop torque is applied to both motors. At the end of the one-shot delay, which
is approximately 100 msec, full torque is removed from the left motor, the brake is applied to the left
motor, and stop torque remains on both motors to take up the slack in the tape until the next command
is received. (Of course the stop torque applied to that motor which has the brake on is doing nothing).
A similar sequence of events occurs when tape movement is in the reverse direction and the
STOP command is received. Hence, after a STOP command occurs, the brake is applied only to the
shaft of the trailing motor, positively stopping the motor shaft to which it is applied. When the tape
is at rest (which is most of the time), the stop torque applied to the motor to take up tape slack is
minimum to avoid excessive heating of the system.
The logical negative levels applied to pins E, F, and H of the G850 SCR Motor Control are
ANDed with the negative BRAKE ENABLE level (C4, D4). This level is negative (B6) only when the
REMOTE/OFF/LOCAL switch is on either REMOTE or LOCAL. When the switch is in the OFF position,
the level is at ground potential and disables both the G850 SCR Motor Control motor circuits and the
brakes (dc +10 and — 15v are still present). This switch position is used when a tape reel is to be
mounted or dismounted and is also recommended when a particular drive is not being used. (On the
older Type TU55 Transports the same 3-position switch was used to interrupt the ac power to the trans-
port and the BRAKE ENABLE level did not condition the G850 circuit. Thus the power itself was
interrupted instead of logically controlling the G850 Motor Control.)
2.2.11 Transport Selected Signal
The negative SELECT level enables another Relay Driver Type W040 (C7) whose output is
sent through a resistor to interface pin K on A5/A6 modules from where it is sent to the control. The
control can use this line to sense whether more than one transport is being selected at one time. Usually
an analog type circuit uses the fact that current could go through the resistor when the relay (transport)
is selected, and, if more than one (or no) transport is selected, causes an error signal. This feature is
not needed when the transport is connected to a 550, 551 , or 552 Control .
Transformer Tl (C3) synchronizes the SCR firing with the 60-cycle waveform of the primary
power. The Thyractor connected across the primary of this transformer protects the diode bridge rectifiers
in the Type G850 Modules.
Table 2-1 Motor and Brake Operation Truth Table
Pin H Pin F Pin E
Pin F Pin E
Complete circuit information for the FLIP CHIP modules in the Type TU55 DECtape Transport
is contained in the replacement schematic engineering drawings (see chapter 5). A list of the FLIP CHIP
modules used in the TU55 is included in engineering drawing PL-TU55-0-5, Module count list. Their
locations are shown on drawing MU-TU55-0-5. These modules are described in DEC Digital FLIP CHIP
Module Catalog, C-105, and in appropriate New Module News Bulletins. Descriptions of non-catalog
modules are contained in the following subsections.
2.3.1 Type R303 Integrating One Shot (Dela y)
This module includes a zero-recovery-time multivibrator and complementary output buffers
(see Figure 2-3 and DEC drawing RS-B-R303). It responds to inputs signals (up to 2 mc) which may occur
at any time either within or outside the delay interval . Successive inputs above a preset frequency can
postpone the delay indefinitely.
-i. X, X, X , X . X «
R401 VARIABLE CLOCK
Figure 2-3 Diagram of R303 Integrating One-Shot
2.3.2 Type W513 Level Amplifier
This module contains six identical level amplifier circuits (see engineering drawing RS-B-
W513). The input levels are received from DEC relay or solenoid driver circuits having outputs which
are between —2 and — 3v when the driver is "on"(1) and are floating when it is "off"(0). An input
current exceeding 1 .2 ma to the level amplifier makes the input potential more positive than — 3.5v and
asserts the output at ground. When the input current is cut off, the input potential returns to approxi-
mately -5v and the output becomes — 3v. Since all circuits are identical, the first circuit in the re-
placement schematic (input at D and output at E) is described.
The circuit is quiescent when no input current is flowing into D. Diodes Dl and D2 clamp
input return Rl at about — 5v. The drop across Dl maintains Q1 cutoff, thus allowing R3 to keep the
Q2 base positive and cut off Q2. D4 clamps output E at the Q2 collector to -3v.
When the input goes more positive to —2 to — 3v it startssupplying current to pin D. It will
back bias Dl and turn on Ql . Q2 base current flows through Ql and R2 so Q2 turns on, bringing E to
ground potential. When the input current at D opens, both transistors cut off again and the circuit is
A resistor is tied to pin T. It is used to simulate relay currents such as those existing in the
555 Relay Transport and therefore allows mixing TU55s and 555s in the same system. The current
through this resistor is measured by an analog circuit at the control to determine whether one or more
transports are being selected and causes an alarm or error signal in case the latter condition happens.
2.3.3 Type G850 SCR Motor Driver
Essentially this module is an SCR switch for the ac voltage supplied to any one of the drive
motors of the TU55. The SCR switch is turned on and off twice during each cycle of the ac power. The
firing point of the SCR is controlled so that it can occur at almost any point along the sinusoidal wave-
form of the ac power; thus controlling the amount of energy that is transmitted to the load (the motor),
and thus achieving control of motor torque. A simplified schematic diagram of this module is shown in
Figure 2-4, a complete schematic in DEC engineering drawing RS-B-G850.
-3 A 'I
5 TO PPjm o-
Figure 2-4 Schematic of G850 SCR Motor Driver
The SCR itself (D15), is connected into a diode bridge so that current always passes though it
in the same direction (an SCR is not a bilateral device). The firing circuit for the SCR incorporates a
unijunction transistor (Q4) and an RC timing network. The capacitor (CI) in this network starts to charge
at the beginning of each half cycle of the ac voltage. When the voltage developed across the capacitor
reaches the firing point of Q4, conduction starts and current flows in the primary of a coupling trans-
former (Tl). Voltage induced in the secondary of Tl controls the firing of the SCR and causes the motor
to operate .
Three input timing circuits determine the trigger timing of the SCR. A -3V level applied to
the two-diode input of these circuits cause the SCR to trigger at a fixed rate or a variable rate determined
by the setting of potentiometers R5 and R9. The fixed input circuit (Q3) triggers the SCR near the
beginning of the sinusoidal wave to produce maximum torque.
A low voltage ac input is supplied to terminals U and V from the secondary at transformer Tl
of the TU55 logic (not Tl of the module). This input voltage is full wave rectified and is clipped by a
Zener diode, but is not filtered. It provides the potential toward which the timing circuit attempts to
charge. Being a stepped down version of the primary ac power which operates the drive motors, this
voltage synchronizes the firing of the SCR to the line frequency. Since the voltage at the end of each
half cycle is zero, the RC network must start charging at the end of each half cycle. Therefore, this
pulsating voltage causes the SCR to fire each half cycle at a point in time measured from the zero
Be very careful when handling this circuit or probing it for measurement
purposes since it contains 115v ac. A short circuit caused by a probe
could produce a high current. Do not tamper with this module without
first disconnecting power from the back terminal.
As stated in chapter 1 , the TU55 can be operated in either a local (manual) or a remote (auto-
matic-programmed) mode. Local operation is accomplished by use of the controls and indicators on the
front panel of the equipment. Remote operation is effected through programmed commands produced by
the stored program of the computer. The following paragraphs contain information applicable to both
modes of operation .
3.2 CONTROLS AND INDICATORS
The functions of the controls and indicators on the control panel in figure 1-1 are summarized
In table 3-1 . Additional information of the action initiated by the controls is given in the following
3.2.1 Operating Notes
When the REMOTE/OFF/LOCAL switch is in the OFF position, the REMOTE indicator does
not light but the TU55 logic still responds to selection, control, and command signals from external con-
trol . Consequently the external control may select and attempt to read or write tape but no tape motion
takes place. In all normal modes, this condition idles both the DECtape control system and TU55 because
no timing channel signal is made available from a motionless tape. When the system is commanded to
write on the timing and mark channels, the timing pulses are generated within the external control per-
mitting the control to operate as if tape motion were available.
3.3 LOADING TAPE
To mount a reel of DECtape on the TU55 after energizing the equipment, proceed as follows
(see figure 1-1 and table 2-1):
To load tape set the REMOTE/OFF/LOCAL switch to the OFF position. This releases the brake
and inhibits the operation of the G850 SCR Motor Driver circuits. Then the hubs are free to be loaded or
unloaded with tape reels. If a new reel is to be mounted, unreel about a foot of tape from it over the
guides and read/write head and wind the tape one or two revolutions onto the right reel. Turn the switch
to either LOCAL or REMOTE as is desired for the next operation.
TABLE 3-1 FUNCTIONS OF CONTROLS AND INDICATORS
Control or Indicator
Reverse tape-motion switch (designated in
figure 1-1 by arrow pointing to the left)
WRITE ENABLED/WRITE LOCK switch
WRITE ENABLED indicator
Address selector (or unit selector)
Provides reverse tape motion (i.e., from right to left)
only if REMOTE/OFF/LOCAL switch is on LOCAL.
Permits external DECtape control system to write in-
formation on the TU55.
Prevents the above writing. If external DECtape con-
trol system is commanded to write on tape during the
WRITE LOCK setting, the control initiates a selection
error flag to notify the program.
The WRITE LOCK setting, however, does not prevent
tape motion when the REMOTE/OFF/LOCAL switch
is on REMOTE.
Lights when WRITE ENABLE/WRITE lock switch is in
the WRITE ENABLE position.
When set to one of the numerals (designating addresses)
and REMOTE/OFF/LOCAL switch is on REMOTE or
OFF, the transport is selected when the line indicated
by the switch wiper corresponds to the computer selec-
tion through the DECtape control. Then the trans-
port responds to command signals from external control
and can assert a write enabling signal to the control .
In addition, all head channels are connected through
the head relay to control bus information lines.
Prevents external DEC control system from selecting
the TU55 .
Permits TU55 to accept command and control signals
from the external DECtape control system; also enables
head relay logic to connect all head channels to con-
trol bus information lines as soon as appropriate trans-
port is selected.
Inhibits operation of the G850 SCR Motor Control
Modules and releases the brakes. Power for the logic
components comes from power supplies associated with
the external control, and therefore the OFF position
does not turn off the +10 and -15v power. The OFF
position is used when loading new tape reels since it
releases the motor hubs. The transport should be set
to OFF when not in use.
TABLE 3-1 FUNCTIONS OF CONTROLS AND INDICATORS (continued)
Control or Indicator
Permits forward and reverse tape-motion switch to
provide tape motion in direction of arrow. Transport
cannot be selected.
Forward tape-motion switch (designated
in figure 1-1 by arrow pointing to the
Lights only when transport is selected by the control.
Provides for motion in the forward direction (i.e.,
from left to right), but only when REMOTE/OFF/
LOCAL switch is on LOCAL. If both reverse and
forward tape-motion switches are pressed simulta-
neously, the forward motion takes place.
Maintenance activities servicing TU55 DECtape Transports require the equipment listed in
table 4-1 , or the equivalent; plus standard hand tools, cleansers, test cables, probes, etc.
TABLE 4-1 RECOMMENDED MAINTENANCE EQUIPMENT
FLIP CHIP module
Triplett or Simpson
630-MA or 260
Series 540 or 580
with Type CA differ-
All four standard
* Head gain and head skew tests require a dual-beam oscilloscope such as Tektronix Type 550 or 551
with a high-gain differential preamplifier for each beam channel, such as Tektronix Type D. However,
these tests are seldom necessary in the field.
The following weekly and monthly preventive maintenance schedules are recommended to
forestall possible failures. Special emphasis is placed on cleanliness of tape handling surfaces because
dust and oxide particles contribute to read errors.
4.2.1 Weekly Schedule
a. Clean entire front panel of tape deck.
b. Clean both surfaces of tape guides and tape head using an appropriate head cleaner
and a soft lint-free cloth.
c. Check proper operation of motor and reel brakes before loading tape by momentarily
pushing forward and reverse tape-motion switches on front panel.
4.2.2 Monthly Schedule
This schedule should be performed on each DECtape once a month when equipment operates
on a standard 8-hour shift and more often when daily operation exceeds one shift. Stagger the check-
outs on individual transports evenly throughout each month.
a. Clean the exterior and interior of the cabinet using a vacuum cleaner and clean cloths
moistened, if necessary, in nonflammable solvent.
b. Clean air filters at the bottom of bay using procedure given in PM section of maintenance
manual for DECtape control system.
c. Lubricate door hinges, casters, etc., with a light machine oil.
d. Inspect equipment for completeness and general condition. Repaint scratched or corroded
e. Make sure each FLIP CHIP module is securely seated in its mounting panel connector.
f . Run a complete exercise and diagnostic routine for each transport so that both the trans-
port and its associated control are exercised in all possible modes and in both directions.
Maintenance programs for this purpose are available from DEC for all computer/control/
transport DECtape systems. Applicable programs are listed in the maintenance manual for
the DECtape control system. Log all errors to provide a malfunction history as an aid to
troubleshooting. If tape is slack or bounces during exercises, perform the adjustment and
calibration procedures in the next subsection.
g. Rotate tapes among all transports and use the read routines of the maintenance programs
to be sure that all transports can read tapes generated by other transports. If each transport
operated satisfactorily instep f, any malfunctions in this step are probably due to head skew.
Head skew can be measured by the procedures given later in the troubleshooting subsection
but cannot be adjusted in the field.
4.3 TAPE TENSION AND TRANSPORT STOP ADJUSTMENT
a. Set the R303 Integrating Delay to about 70 ns. Pin D in A04 of the delay is varied
with the aid of the trimpot mounted on the module.
Initiation of the delay is made by pressing momentarily on either the forward or reverse
switch when the REMOTE/OFF/LOCAL switch is on LOCAL. No tape reel is needed for
b. Mount a tape reel on the transport. Press the forward switch and watch the tape move-
ment. The trailing torque of the left motor should be adjusted on the G850 Module A1 1
trimpot (as shown in figure 4-1). If the torque is too great it will impede the movement at
the tape; and if too little, flapping or loose contact with the read write head will result.
The technician who maintains the equipment might develop a "touch" so that by pressing
his finger over the tape at the trailing reel he can tell if the tape is too tight or loose,
simply by the stiffness of the tape while in motion.
c. Repeat same procedure for reverse tape movement by pressing reverse switch. This
time the pot adjustment is done on module A12.
d. Press forward switch for a second or two and release it; the tape action while stopping
should be smooth. If bouncing occurs, Increase the stopping torque until bounce just
disappears. This stop torque is maintained when the transport is not operating (which is
most of the time) and excessive torque merely results in overheating the motor and hence
the whole equipment. Torque adjustment is made on the appropriate pot (figure 4-1) on
e. Repeat same procedure when pressing the reverse switch, watching for the stop when
tape movement is in reverse. Adjust pot on module Al 1 .
POT FOR STOP
(OR AT REST)
Figure 4-1 Module Adjustment Trimpots
HEAD OUTPUT CHECK
This check is used to determine whether the read-head is developing the proper read signals,
If the read-head performance is not satisfactory, the head should be replaced. No attempt should be
made in the field to repair or even change head position by loosening the brackets that hold it to the
plate. Although DEC personnel are authorized to replace heads in the field, the replacement involves
realignment and usually requires the return of the transport to DEC, Maynard. The following diagnostic
procedure can be performed with any single-channel oscilloscope having a high-gain differential pre-
amplifier capable of handling signals in the millivolt range over a bandwidth of to 60 kc , (Tektronix
Type D Plug-in Unit).
a. Mount a DEC certified tape on the TU55 Transport. Move tape manually in a forward
direction until approximately one half of the tape is on each reel.
b. Disconnect the 22-pin Amphenol head connector from rear of Head-Relay Module G851
located at the extreme right of the mounting panel seen from the rear. Attach the two probes
of the differential amplifier to pins B and C of this connector and attach the ground clip to
D. Set the oscilloscope to sync internal, and set the vertical preamplifier to 5 mv/cm
(0.5 mv/cm if an XT probe is used).
An alternative method is to cause a closure of the information relays by grounding (with the
aid of a clip) pin E of the G851 Module at location AB1 . This eliminates disconnecting the
plug procedure, as outlined above, by making it possible to monitor the same plug points on
the wiring panel front. The probes are now attached to pins AF and AH of the same FLIP CHIP
socket AB1 . The gound clip is attached to pin C.
c. Move tape forward and reverse using forward tape-motion switch and observe that the
read- head waveform is about 10 to 12 mv peak-to-peak.
d. Repeat step c, attaching the probes to pins F-H, P-R, U-V, and Y-Z. These signals may
not look sinusoidal (as they do for pins B-C), but the peak-to-peak amplitude should measure
the same as in step c. The same alternative exists here as in step b. The pin pairs to monitor
now are: AN-AP, AV-BD, BH-BJ, and BN-BP.
4.5 HEAD-SKEW CHECK
Although the head-skew is not adjustable in the field, the following check should be performed
when unusual numbers of read errors are observed on an individual transport engaged in reading tapes
generated by several other transports. The procedure requires a dual-beam oscilloscope with two high-
gain differential preamplifiers for each scope channel (as outlined in the equipment needed,4.1).
a. Write a timing track on a scratch tape using one of the DECtape utility routines.
b. Rewind tape back onto the left reel. Dismount the left reel from the left hub and mount
it on the right and mount the empty reel on the left hub. Thread tape over the head and onto
the left reel. Move tape in reverse to fill up the left reel. The tape is now with its oxide
facing up away from the head.
c. Disconnect the 22-pin Amphenol head connector from rear of Head Relay Module G851 .
This module is located at the extreme right of the mounting panels, seen from the rear.
d. Connect the two upper-scope channel probes to head connector pins A and C and their
ground clips to the chassis. Connect the two lower-scope channel probes to head connector
pins A and D and their ground clips to the chassis. Set both channel preamps to A-B (the
e. Move tape forward by using the forward tape-motion switch. Adjust channel gain so
that each waveform has the same peak-to-peak voltage. Sync on the upper beam and take
a reading of the phase difference between the signal peak at pins A and D (track 10) with
respect to the signal peak at pins A and C (track 1). Be sure to record whether the track
10 signal leads or lags the other and the amount. Make all readings as near the center of
the tape length as possible. Skew readings greater than 5 psec indicate possible incompati-
bility among transports. Probably the head has become misaligned through physical damage.
Then the transport must be returned to DEC, Maynard, for realignment.
f. Remove all probes, reconnect the Amphenol connector to the rear of the G851 Head
Relay Module, and rewind the tape to the original state.
NOTE 1: It is possible here also to use the alternative method of monitoring
the channel waveforms (see alternate method, section 4.4b) by energizing
the relay at the G851 Module by grounding pin E. By doing so probe clips
can be attached to the wiring panel pins at location Al . The pins are D-H
for one probe and D-J for the other.
NOTE 2: In some heads (manufacturer, GJM) use pin F instead of pin H on
the FLIP CHIP socket. On the Amphenol plug at the back use pin B instead
of pin C.
4.6 WRITE ENABLE CIRCUIT CHECK
a. Set the WRITE ENABLE/WRITE LOCK switch to WRITE position and check that the WRITE
ENABLE indicator lights. Also check that a ground level is present at terminals A5L and
b. Set the switch to the WRITE LOCK position and check that the WRITE ENABLE indicator
is extinguished. Also, check that level at terminals A5L and A6L is — 3v. Check transmission
of this level from terminal A5L to the DECtape control system.
The forward and reverse tape-motion switches on the front panel of the 11)55 are useful in
checking the operation of the unit and in assuring that the tape-motion mechanism is working properly.
It is suggested that the first step in troubleshooting should be a check of the modular logic circuits.
For this check, the primary ac power cable should be disconnected from the plug at the back
of the transport to prevent movement of the tape drive motors. Set the REMOTE/OFF/LOCAL switch to
the LOCAL position. Then simulate the commands to GO FORWARD and GO REVERSE by pressing the
appropriate switch. When the switch is released, the STOP command is initiated to energize the left
motor shaft brake (the brake coil is energized from the — 15v supply). Under these conditions, use an
oscilloscope or a voltmeter to compare the command signals throughout the logic circuits against those
In table 3-1 .
This operation is checked by monitoring the terminals of the Type G850 Modules (AT 1 and
A12) and the Type W040 Modules (B12) with an oscilloscope of voltmeter to verify proper signal levels
for all operating conditions of the MOTION and DIRECTION flip-flops (B08). Then inspect the motor
hubs to check for proper operation of the brakes. This is accomplished by assuring that the proper hub
stops when called upon to do so by the command logic (activated by operation of the front panel tape-
If the truth table cannot be verified, continue troubleshooting by signal tracing, monitoring
the signal levels at different NAND gate inputs and outputs and working back toward the control flip-
flops until the cause of the malfunction is found. If the truth table can be verified, restore the primary
ac input power, install a reel of tape on the drive mechanism, and repeat the truth table verification
procedure for proper tape movement and end stopping. If the malfunction still exists, check the
following sources of trouble:
a. Transformer Tl secondary is not presenting stepped down ac power to terminals U and V
of the modules A1 1 and A12 (the Type G850 SCR Motor Driver Modules).
b. Type G850 Modules are defective.
c. Electromechanical system (e.g. reel motor) rather than the control logic.
Table 4-2 lists the recommended spare parts for 11)55 Transport systems. A R-series module
in the TU55 may be replaced by an S-series module with the same number. All items are available at
DEC, Maynard, but may be ordered from the other firms listed.
TABLE 4-2 RECOMMENDED SPARE PARTS
0-1 1-1 -A
SCR motor driver
Integrating one shot
Rotary selector switch (address selector)
Rocker switch (REMOTE/OFF/LOCAL)
Rocker switch (WRITE ENABLED/LOCK)
Rocker switch (forward and reverse tape-
motion) (with off-nonmomentary on circuit)
Capacitor (reel motor), 7 mfd, 300 wvdc
Torque motor, 70-frame, SP spec
Friction brake, FB-181-23
* These modules are contained in the basic PDP-8 or the TC01 DECtape Control, so duplicate spare
parts are not required if the TU55 is part of this system configuration.
** This module is contained in the TC01 , so a duplicate is not required if the TU55 is used with a TC01
DECtape Control .
*** This module is required only if the TU55 is not used with a TC01 DECtape Control.
This section contains reduced copies of the engineering drawings, (see Pertinent Documents
chapter 1) required for understanding and maintaining the Type TU55 DECtape Transport. These drawings
are in addition to the complete set of full-size drawings forwarded with each TU55 . Only the full size
drawings should be used by maintenance personnel for work on the units. The full-size drawings show
variations peculiar to an individual installation.
5.2 CIRCUIT SYMBOLS
The block schematics of DEC equipment ore multipurpose drawings that combine signal flow,
logical function, circuit type and physical location, wiring, and other pertinent information. Individual
circuits are shown in block or semiblock form, using special symbols that define the circuit operation.
These symbols are similar to those appedring in the FLIP CHIP Module Catalog but are often simplified.
Figure 5-1 illustrates symbols used in DEC engineering drawings.
5.3 LOGIC SIGNAL SYMBOLS
DEC standard logic signal symbols are shown at the input of most circuits to specify the enabling
conditions required to produce a desired output. These symbols represent either standard DEC logic levels
or standard FLIP CHIP pulses.
5.3.1 Logic Levels
The standard DEC logic level is either at ground (0 to — 0.3v) or at — 3v (—2.5 to — 3.5v).
Logic signals generally have mnemonic names which indicate the condition represented by assertion of the
signal. An open diamond ( > ) indicates that the signal is a DEC logic level and that ground represents
assertion; a solid diamond ( ► ) indicates that the signal is also a DEC logic level and that — 3v repre-
sents assertion. All logic signals applied to the conditioning level inputs of diode-capacitor-diode gates
must be present for a minimum of 400 nsec before an input pulse will trigger operation of the gate.
5.3.2 FLIP CHIP Pulses
Two types of pulses, R series and B series, are used in FLIP CHIP circuit operation. The pulse
produced by R-series modules starts at — 3v, goes to ground (— 0.2v) for 100 nsec, then returns to — 3v.
This pulse is shown in figure 5-2.
GROUND LEVEL PULSE
LEVEL TRANSITION USED AS A PULSE
OR TRIGGERING ON THE LEADING EDGE
OF A GROUND LEVEL
TRIGGERING ON THE TRAILING EDGE
OF A PULSE
-15V LOAD RESISTOR CLAMPED AT -3V
[ a] or 1 r~1
PNP TRANSISTOR INVERTER
GROUND -LEVEL NAND, NEGATIVE-LEVEL
NOR DIODE GATE
GROUND-LEVEL NOR, NEGATIVE -LEVEL
NAND DIODE GATE
Figure 5-1 DEC Symbols
CAPACITOR -DIODE GATE, POSITIVE OR NEGATIVE
INDICATED BY POLARITY OF THE INPUTS.
1. PULSE INPUT
2. CONDITIONING LEVEL INPUT
3. PULSE OUTPUT
I. PULSE INPUT, POLARITY INDICATED
BY INPUT SIGNAL
2,3. TRANSFORMER -COUPLED PULSE
OUTPUT. EITHER TERMINAL MAY
6 7 A 6 7
Z 5 4
FLIP-FLOP (MOST FLIP-FLOPS HAVE ONLY SOME
OF THE FOLLOWING):
I. DIRECT- CLEAR INPUT
2. GATED-CLEAR INPUT
3. DIRECT-SET INPUT
4. GATED-SET INPUT
5. COMPLEMENT INPUT
6. OUTPUT LEVEL, -3 V IF 0,OV IF 1
7. OUTPUT LEVEL, OV IF 0, -3V IF 1
8. CARRY PULSE OUTPUT, UPON BEING CLEARED
DELAY (ONE -SHOT MULTIVIBRATOR)
I. INPUT PULSE
2. OUTPUT LEVEL, -3 V DURING DELAY
3,4. TRANSFORMER -COUPLED PULSE
OUTPUT. EITHER TERMINAL MAY
Figure 5-1 DEC Symbols (continued)
Figure 5-2 FLIP CHIP R-Series Pulse
The B-series negative pulse is 2.5v in amplitude and 40 nsec in width as shown in figure 5-3
If this pulse is applied to the base of an inverter, the inverter output will be a narrow pulse, similar in
shape to the R-series standard pulse. The B-series positive pulse, which goes from ground to+2.5v,
the inverse of the B-series negative pulse.
pfr- WIDTH -*j
Figure 5-3 FLIP CHIP B-Series Pulse
Standard EIA components as specified in table 5-1 can replace most DEC semiconductors used in
modules of the TU55 DECtape Transport. Exact replacement is recommended for semiconductors not listed.
TABLE 5-1 SEMICONDUCTOR SUBSTITUTION
efe t>*/ fi
Solid State DECtape Transport BS-D-TU55-0-2, Rev J
* LOCATIONS FOR UNIT TO UNIT BUS CONNECTIONS.
I. FOR MODULE COUNT SEE PARTS LIST
Module Utilization List MU-D-TU55-0-5
I 2 3 A 5 6 7 8 9 10 II 12
I. JUMPER A#4C TO A Of 7C
Bus Bar for TU55 WD-D-TU55-0-4
GND AC O—
RELAYS ARE WHEELOCK
REED, 260 SERIES 3A
Diode Cluster RS-B-R002
UNLESS OTHERWISE INOICATEDt
RESISTORS ARE l/4W v 8*
DIODES ARE 0-664
TRANSISTOR! ARE DEC 3639B
PRINTED CIRCUIT "REV. FOR
DOL BOARD IS SIA
oo — W-
■o — W-
: L D-662
15,000 > 7,600
>I6,000 > 7,600
UNLESS OTHERWISE INDICATED:
RESISTORS ARE I/4W; 5%
PRINTED CIRCUIT REV. FOR
DGL BOARD IS SIB
Diode Gate RS-B-R111
Dual Flip-Flop RS-B-R202
IMLEM OTHOIWK INOKATEO
■KMTOM «W IMW.10%
DKIOCt ME 0*962
■TOIIt ME ' DEC MMt PARTS UtT A-*l~*Ml-0-0
Integrating One-Shot RS-B-R303
UNLESS OTHERWISE INDICATED;
RESISTORS ARE I/4W; 10%
DIODES ARE D-664
Solenoid Driver RS-B-W040
UNLESS OTHERWISE INDICATEO
TRANSISTORS ARE DEC 2694-16
DIODES ARE 664
RESISTORS ARE 10,000; I/4W, 10 %
RESISTORS ARE 1/4 W, 10%
NOTE TO BE USED WHEN SOlIO STATE DEC TAPE TU56 IS
CONNECTEO TO A CONTROL THAT HAS. RELAY DRIVERS. (550,351,552)
.01 -I- i 1*0
MFD I i 2W
Level Amplifier RS-B-W51 3
DIGITAL EQUIPMENT CORPORATION □ MAYNARD , MASSACHUSETTS
Printed in U.S.A.